1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine and a printer. More particularly, the invention relates to an improvement of an image forming apparatus of the type in which a sheet bearing an image, which is formed by an image forming part of a main body of the image forming apparatus, is discharged to a sheet receiving part by way of a sheet discharging device.
2. Background Art
A Xerography-based copying machine will be described as a typical example of the image forming apparatus of this type. In the copying machine, a toner image is formed on an image bearing member, such as a photoreceptor drum, and the toner image is transferred onto a sheet, such as a printing paper, directly or through an intermediate transfer body. Thereafter, the toner image, not yet fixed, is fixed on the sheet, and is discharged onto a discharging tray as the sheet receiving part.
In the image forming apparatus thus constructed, a sheet discharging device is provided at a position near an output part of a sheet transport path. The sheet discharging device discharges the sheet bearing the fixed image, which has passed a fixing device, into the discharging tray. This type of sheet discharging device usually includes paired discharging rolls, and these rolls nip the sheet there between and transports the sheet in a nipping state.
When a toner image is formed on an OHP sheet at high speed, toner image fusion was found on the OHP sheet having an image formed thereon placed on the discharging tray. It was confirmed that the toner image fusion leads to image defects, such as image irregularity and image peeling.
This kind of technical problem is due to the fact that the OHP sheets are stacked while storing up therein the heat applied to the sheets by the fixing device. Therefore, it is readily understood that the problem can be solved in a manner that the OHP sheets having undergone fixing process are sufficiently cooled and then stacked.
Some approaches to the problem are proposed. In one approach, a cooling device is located downstream of the fixing device. After pass the fixing device, the OHP sheets or the like are forcibly cooled and then discharged. In another approach, the sheet-to-sheet span is increased by thinning out sheets successively transported to secure a sheet cooling time, although the copy productivity is decreased.
In the former approach, a large cooling device is required to secure a large cooling effect. Accordingly, the device cost is considerably increased. In a case where the sheet transport path ranging from the fixing device to exit rolls is curved, when the sheet is cooled by the cooling device, the sheet tends to remain curled to thereby adversely influence the post-processing of sheets.
In the latter approach, the sheets are transported in thinned-out manner. Accordingly, the copy productivity is considerably reduced. A time from the moment that the sheet leaves the fixing device to the moment when it arrives at the discharging tray and stacked therein remains unchanged. Therefore, the solution approach is insufficient in gaining such effects that the amount of heat stored up in the OHP sheets being stacked in the discharging tray is reduced, thereby eliminating image defects.
Such technical problems are found not only in the OHP sheets but also in special sheets, e.g., coated sheets.
Accordingly, an object of the present invention is to provide an image forming apparatus which, with simple construction, is capable of preventing image defects, which arise from the fusing of image forming materials (e.g., toner) of sheets to be stacked on a sheet receiving part, while minimizing the lowering of copy productivity.
The present invention provides an image forming apparatus which includes, as shown in FIG. 1, a sheet discharging device 3 located near the output part of a sheet transport path 2 within a main body 1 of the image forming apparatus and discharges a sheet S having an image formed thereon by an image forming part 11 within the main body 1 of the image forming apparatus to a sheet receiving part 4, wherein the sheet discharging device 3 comprising: a discharging member 5, disposed near the output part of the sheet transport path 2, for transporting the sheet in a nipping manner; a drive force transmitting mechanism 6 for driving the discharging member 5 to rotate; and a discharge control unit 7 for controlling the drive force transmitting mechanism 6 when the sheet S passes the discharging member 5 to thereby delay a time taken for sheet discharging by the discharging member 5.
In the thus implemented technical idea, the image forming part 11 involves various types of image forming systems, if those are capable of forming an image on a sheet S, such as a Xerography-basis image forming system, electrostatic image forming system, and an ink jet image forming system.
The image forming part 11 shown in FIG. 1 employs the Xerography-basis image forming system. The image forming part 11 includes an image forming engine 8 for forming an image on a sheet S on the sheet transport path 2, which is supplied from a sheet supplying unit 12, a transfer member 9 for transferring the image formed by the image forming engine 8 onto the sheet S, and a fixing unit 10 for fusing and fixing the image transferred onto the sheet S.
Particularly, in the embodiment in which the image forming part 11 includes the fixing unit 10, the fixing unit 10 discharges the sheets S bearing the images fused and fixed. When the sheets S are stacked while storing up heat, the technical problem (image irregularity, the peeling of an image) appears remarkably. In this respect,: the present invention effectively operates in the embodiment.
Even in an image forming part not including the fixing unit 10, for example, the one based on an ink jet image forming system, when the sheets S are stacked before the ink is dried, the technical problem (image irregularity, the peeling of an image) arises. Therefore, the present invention also effectively operates also in this case.
Where the sheet discharging device 3 is provided near the output part of the sheet transport path 2 in the main body 1, it may be incorporated into the main body 1 of the image forming apparatus as its constituent part or as a separate unit.
The sheet receiving part 4 may take any form if it is capable of receiving sheets S discharged from the sheet discharging device 3. The sheet receiving part 4 may directly be installed on the top part of the main body 1 of the image forming apparatus or it may be installed on the side of the main body 1 in the form of a tray member.
Usually, the discharging member 5 is constructed with a pair of rolls 5a and 5b, and one of the rolls serves as a drive roll and the other, as a follower roll. Further, the discharging member 5 may be constructed with paired belts, or a combination of a roll and a belt.
The drive force transmitting mechanism 6 may take any suitable form if it is capable of transmitting a driving force from a drive source (which may be a drive source dedicatedly provided or another drive source which is used for another driving purpose) at an appropriate reduction ratio or speed increase ratio.
The discharge control unit 7 may include any means capable of controlling the drive force transmitting mechanism 6 so as to temporarily delay a time taken for the discharging member 5 to discharge the sheet S. The discharge control unit 7 may be designed so as to control the coupling/decoupling of the drive source to the drive force transmitting mechanism 6, or to control a combination of drive force transmitting members.
The control by the discharge control unit 7 may be any control if it is capable of temporarily delaying a time taken for the discharging member 5 to discharge the sheet S. For example, it may set the sheet discharging speed by the discharging member 5 at a slow speed. A preferable control by the discharge control unit 7 is to temporarily stop the discharging member 5.
The control may be implemented such that it temporarily stops the drive/rotation of the discharging member 5 when the sheet S passes the discharging member 5.
If the sheet S being discharged is stopped, there is a chance that the user mistakenly pulls out the sheet S being stopped. To lessen occurrence of such a chance, when the temporarily stopping/discharging process is carried out, it is preferable that when the drive/rotation of the discharging member 5 is temporarily stopped, the discharge control unit 7 releases the discharging member 5 from its restraint by the drive force transmitting mechanism 6, whereby the sheet S being nipped by the discharging member 5 is allowed to freely be pulled out.
If so done, even when the user pulls out the sheet S being stopped, there is less chance that the discharging member 5 and the sheet S are both damaged.
Another control for the temporarily stopping/discharging process by the discharge control unit 7 is to set a sheet-to-sheet span to be wider than a normal sheet-to-sheet span.
In the embodiment, a delay amount of the sheet discharging time is secured by the widening of the sheet-to-sheet span. A preferable embodiment to widen the sheet-to-sheet span is to widen the sheet-to-sheet span so that a time from an instant that the trailing edge of the sheet S passes the fixing unit 10 till the trailing edge passes the discharging member 5 exceeds a predetermined value of time necessary for cooling the sheet S.
The discharge control unit 7 may carry out the temporary delaying process for all the sheets S, but it is preferable that the discharge control unit 7 selects the temporary delaying process of the time taken for the discharging member 5 to discharge the sheet S, according to a sheet kind.
In this case, the discharge control unit 7 selectively carries out a temporary delaying process of the time taken for the discharging member 5 to discharge the sheet S, according to a sheet kind.
In some kinds of sheets, there is no necessity of executing the temporary delaying process. In such a case, in the embodiment, the temporary delaying process is not carried out to thereby eliminate useless execution of the temporary delaying process.
The word xe2x80x9csheet kindxe2x80x9d involves sheet size as well as sheet material.
A typical example of sheet S, which creates the technical problem to be solved by the invention, is an OHP sheet. Therefore, when applied to the OHP sheet, the present invention is useful in particular.
In this case, it is preferable that the discharge control unit 7 selectively carries out a temporary delaying process of the time taken for the discharging member 5 to discharge the sheet S, at least under a condition that the sheet S is an OHP sheet.
Even in the case of the OHP-sheet, in a slip sheet mode in which a plain paper is inserted as a slip sheet between the OHP sheets, the technical problem to which the invention is directed does not arise since the slip sheet is inserted between the OHP sheets.
Accordingly, when a slip sheet mode is selected, the discharge control unit 7 preferably prohibits the temporary delaying process of the time taken for the discharging member 5 to discharge the sheet S, even under a condition that the sheet S is an OHP sheet.
Further, it is preferable for the discharge control unit 7 to variably set a delay time of a time taken for the discharging member S to discharge the sheet S, according to a sheet kind.
This embodiment can handle with such a situation that a time necessary for cooling the sheet S is different depending on a sheet kind.
Preferably, the temporary delaying process to be carried out by the discharge control unit 7 is selected according to a job condition.
In a preferable example of such, the discharge control unit 7 selectively carries out a temporary delaying process of a time taken for the discharging member 5 to discharge the sheet S, according to the set number of sheets in one subject job.
In some set numbers of sheets in a job to be processed (the smaller number of sheets), there is no necessity of executing the temporary delaying process. In such a case, the temporary delaying process is not carried out to thereby eliminate useless execution of the temporary delaying process, in the embodiment.
The temporary delaying process, carried out by the discharge control unit 7, is not always applied uniformly to the entire one subject job, but may be applied selectively to a part of the one subject job.
This may be implemented such that the discharge control unit 7 selectively carries out a temporary delaying process of a time taken for the discharging member 5 to discharge the sheet for the set number of sheets in one subject job, which is larger than a predetermined number of sheets and for those sheets subsequent to a specified number of sheets.
There is a case where the temporary delaying process is not required for some numbers of sheets in one job (a first sheet, first several number of sheets, or the like). In such a case, the temporary delaying process is not carried out to thereby eliminate useless execution of the temporary delaying process, in the embodiment.
The temporary delaying process by the discharge control unit 7 may be executed every single job, preferably for the continuous job process.
This may be implemented such that under a condition that a certain job to be processed is continuous to one subject job, the discharge control unit 7 selectively carries out a temporary delaying process of a time taken for the discharging member 5 to discharge the sheet S according to the set number of sheets, which is the sum of the set number of sheets in the preceding job and that in the subsequent job.
The jobs of the continuous job process may be considered to be equivalent to a succession of jobs.
The term xe2x80x9cpreceding jobxe2x80x9d involves a plurality of jobs as well as a single job.
In the case of the continuous job process, it is preferable to consider a time between the adjacent jobs.
In this case, the discharge control unit 7 selectively carries out a temporary delaying process of a time taken for the discharging member 5 to discharge the sheet S, in consideration of a time between the adjacent jobs additionally.
In the embodiment, the time between the jobs is taken into consideration for the following reason. When the time between the jobs is long, there is a case where though those jobs are contained in the continuous job process, such jobs are merely single jobs and it can be hardly conceived that those jobs are successive jobs. Therefore, it is necessary to judge whether or not the jobs are substantially successive continuous jobs.
In the present invention, it is preferable that the discharge control unit 7 is capable of varying the set number of sheets and a specific number of sheets.
Further, the discharge control unit 7 is preferably capable of removing a temporary delaying process of a time taken for the discharging member 5 to discharge the sheet S.
If so done, the user who does not need the temporary delaying process may be put out of the control on the temporary delaying process.
The image forming apparatus may include a forcibly cooling device for cooling a surface of a sheet S, if necessary.
The forcibly cooling device may be a heating/cooling element, such a cooling fan or a Peltier element.
The forcibly cooling device may be located at the sheet transport path 2 ranging to the discharging member 5, or located so as to cool a surface of a sheet S nipped by the discharging member 5.
In the embodiment, it is satisfactory for the forcibly cooling device to exhibit an auxiliary cooling function. Accordingly, the cooling device itself may be small in size, and not increased in cost.
According to another aspect of the invention, there is provided an image forming apparatus which, as shown in FIG. 1, includes a sheet discharging device 3 located near the output part of a sheet transport path 2 within a main body 1 of the image forming apparatus and discharges a sheet having an image formed thereon by an image forming part 11 within the main body 1 of the image forming apparatus to a sheet receiving part 4, wherein the sheet discharging device 3 comprising: a discharging member 5, disposed near the output part of the sheet transport path 2, for transporting a sheet S in a nipping manner; a plurality of discharging paths A and B of different lengths provided within a range from the image forming part 11 to the discharging member 5 of the sheet transport path 2; and a discharge control unit 7 for selecting discharging path A or B according to a sheet kind or job condition.
In FIG. 1, the sheet transport path 2 of the discharging path B is selected to be longer than that of the discharging path A.
In the embodiment, the discharging path A or B is selected according to the sheet kind or job condition.
For example, when an OHP sheet is used, the discharging path B of which the sheet transport path 2 is long is used to gain a time till the sheet S is discharged to the sheet receiving part 4.
Within a range from the image forming part 11 to the discharging member 5, the discharging paths A and B may be provided separately. A part of the discharging paths A and B may be commonly used.
The long discharging path B may be additionally provided. To simplify the device construction, the long discharging path B preferably is a sheet transport path used for another purpose, which is utilized in the form of a bypass.
In this case, the sheet transport path 2 used for another purpose may be a reversing transport path, both-side recording transport path, a transport path to the finisher, or the like.
In the embodiment, the discharging member 5 may be located at one discharging port or a plurality of discharging ports.
Where a plurality of discharging ports are used, viz., discharging members 5 of the discharging paths A and B are located at different positions, the sheet receiving parts.4 are separately used, and different sheets S are discharged into those sheet receiving parts 4 according to the sheet kind and job condition.
Also in the embodiment, a forcibly;cooling device for cooling a surface of a sheet may additionally be provided.
In this case, a forcibly cooling device for cooling a surface of a sheet is provided at the discharging path or discharging paths.
Where it is impossible to secure a sufficient length for the discharging path B, for example, the forcibly cooling device is installed on the discharging path B to support the cooling of the sheet S. Where it is possible to secure a sufficient length for the discharging path B, for example, the forcibly cooling device is installed on the discharging path A.
Where the sheet transport path 2 used for another purpose is used for discharging path B, and is a sheet return path for the both-sided recording or multiple recording purposes, a forcibly cooling device is provided at the sheet return path.
When the forcibly cooling device is installed at the sheet return path, it cools the sheet S to be discharged into the sheet receiving part 4. Further, when the sheet S bearing an image recorded thereon returns to the image forming part 11, it is cooled again. As a result, it is effectively avoided that temperature within the main body 1 rises.